A plane type image display device is applied to various fields such as a TV, a personal computer, a cellular phone (mobile) and the like, and is rapidly expanding in the market. In such plane type image display devices, a protective panel may be disposed on a display panel or a touch panel to prevent the display panel or the touch panel from being damaged. Conventionally, an image display panel and a protective panel were laminated with a space disposed between them, whereby to enhance shock-absorbing characteristics, and thus to prevent scratch or crack of the image display panel. However, it has a problem in that reflection occurs in the interface of the protective panel and the space, leading to a decrease in image visibility, particularly in outdoor image visibility. Furthermore, it also has a problem in that it is difficult to realize a thin body due to disposition of the space.
As a method of improving both thinning and visibility, a method has been suggested, in which an image display panel and a protective panel are directly laminated via a transparent adhesive agent or adhesive sheet (see those described below).
For example, Patent Reference 1 discloses an adhesive sheet that has a first pressure-sensitive adhesive layer, second pressure-sensitive adhesive layers formed on the two surfaces of the first pressure-sensitive adhesive layer, and release films adhering to adhesive surfaces of the two pressure-sensitive adhesive layers, in which the first pressure-sensitive adhesive layer is formed with use of a pressure-sensitive adhesive agent having viscoelastic characteristics such as of 5×103 to 5×105 Pa of a storage modulus G′(1 Hz) at 20° C. measurement temperature and 1 Hz frequency, and 5×101 to 5×103 Pa of a storage modulus G′(10−7 Hz) at 20° C. standard temperature and 10−7 Hz frequency, and the second pressure-sensitive adhesive layer is formed with use of a pressure-sensitive adhesive agent having viscoelastic characteristics such as 1×104 to 5×106 Pa of a storage modulus G′(1 Hz) at 20° C. measurement temperature, and 1×104 to 1×106 Pa of a storage modulus G′(10−7 Hz) at 20° C. standard temperature, which is an adhesive sheet that does not require temporary bond by heating or high temperature and high pressure treatment with an autoclave, and can bond glass plates at room temperature.
Furthermore, Patent Reference 2 discloses a method of forming, before ultraviolet cross-linking, an adhesive layer having a low cohesion force (5×103 to 5×105 Pa of a storage modulus G′ at 20° C. measurement temperature and 1 Hz frequency, and 5×101 to 5×103 Pa of a storage modulus G′ at 20° C. standard temperature and 10−7 Hz frequency), which includes bringing panels into intimate contact with each other via the adhesive layer without generating residual air bubbles therebetween; and irradiating ultraviolet rays (UV) after the intimate contact onto the surface of the protective panel to cure the protective panel so that the protective panel has viscoelastic characteristics of 1×104 to 1×106 Pa of a storage modulus G′ at 20° C. measurement temperature and 1 Hz frequency, and 1×104 Pa or more of a storage modulus G′ at 20° C. standard temperature and 10−7 Hz frequency.
Furthermore, Patent Reference 3 discloses a constitution in which two kinds of adhesive layers having different cohesion forces are disposed on both sides of a gas barrier base material, in which panels are brought into intimate contact with each other without generating residual air bubbles by making use of a first adhesive layer having a low cohesion force, and attack of volatile components from a protective panel is blocked by the gas barrier base material and a second adhesive layer having high cohesion force.
Patent Reference 1: Japanese Patent Application Laid-Open (JP-A) No. 2001-234129
Patent Reference 2: JP-A No. 2002-348150
Patent Reference 3: International Publication No. WO2006-112311
Such conventionally disclosed adhesive agent or adhesive sheet has such a problem that when a laminated body obtained by laminationing via the adhesive agent or adhesive sheet is cut, cut edges become sticky after a period of time and thus adheres to a production facility, which becomes a trouble in the production process, and invite foreign substances, dust and the like to adhere thereto, which becomes a cause of contamination.
Furthermore, for example, in a display screen of a mobile phone and the like, a constitution was sometimes adopted in which a polarized film and the like were laminated on a liquid crystal panel display (LCD), and on top of that, a protective panel made of plastic was laminated via an adhesive agent or adhesive sheet as shown in FIG. 1. At this time, a black print portion (5 μm to 20 μm or so in thickness) was normally provided in the margins on the back side of the protective panel. In such a case, if the adhesive agent does not sufficiently have ability of encapsulate features of an uneven portion of a stepped portion that is formed in the margin of the black print portion, air bubbles remain leading to a decrease in screen visibility. Furthermore, if the protective panel is made of plastic, gas (referred to as outgas) is generated from the protective panel. Therefore, if the adhesive agent or adhesive sheet does not have enough adhesive force and cohesion force to oppose the pressure of this gas, the gas remains in the adhesive agent or adhesive sheet, and thus if the temperature becomes high, the residual gas foams leading to a decrease in screen visibility.
As a countermeasure to such outgas, for example, as described in Patent Reference 2 above, suggested is a method of forming an adhesive agent having a low cohesion force, which includes bringing panels into intimate contact with each other without generating residual air bubbles, and irradiating ultraviolet onto the protective panel to cause cross-linking. However, in this method, the cross-linking needs to be performed after bonding of the protective panels, and thus the method has a problem from the point of view of the productivity. Furthermore, for example, as described in Patent Reference 3 above, suggested is a method which includes disposing an outgas barrier layer. However, this method has problems such as an increase in thickness or weight, and a decrease in transparency.
Therefore, an object of the present invention is to provide a novel transparent adhesive sheet, with which the cut edge is not sticky over time when a laminated body that has been bonded via the adhesive sheet is cut, which can adhere no residual air bubbles, even when the surface of an adherend has unevenness of 5 μm to 20 μm or so, and further which can adhere without foaming under high temperature environment, for example, 80° C. or so, even when the adherend is a material such as plastic that generates outgas.
The present invention suggests a transparent adhesive sheet that is constituted to have one or more layers respectively of a first adhesive layer and a second adhesive layer that exhibit different viscoelastic behaviors from each other, and has an integrated structure whereby these layers are laminated and has value of dynamic shear storage modulus G′ when measured with a 1 Hz frequency temperature dispersion which is within ranges described below:                2×104 to 5×105 Pa for G′(20° C.)        1×104 to 1×105 Pa for G′(150° C.)        
With an adhesive sheet imparted with the viscoelastic behaviors as described above, when a laminated body that has been bonded via the adhesive sheet is cut, the cut edge is not sticky over time, and the adhesive sheet can adhere with no residual air bubbles, even when the surface of an adherend has unevenness of 5 μm to 20 μm or so, and further can adhere without foaming under high temperature environment, for example, 80° C. or so, even when the adherend is a material such as plastic that generates outgas.
In order to adhere without generating residual air bubbles when the surface of an adherend is uneven, generally a soft and easily wettable adhesive agent or adhesive sheet is used. However, simply only with softness of the adhesive agent or adhesive sheet, the cut edge becomes sticky, or the adhesive agent or adhesive sheet does not have a cohesion force to oppose the gas pressure of the outgas, and thus the gas remains inside, and the residual gas foams when the temperature becomes high, leading to a decrease in screen visibility. On the other hand, with a hard adhesive agent or adhesive sheet, when the surface of an adherend is uneven, air bubbles remain, leading to a decrease in visibility.
Therefore, the present invention allows an adhesive sheet to have a combination of incompatible properties, that is to say, a property of the cut edge not being sticky over time after the cutting, a property of adhering without generating residual air bubbles even when the surface of an adherend is uneven, and further a property of sufficiently withstanding outgas and thus adhering without foaming under high temperature, not by means of simply manufacture of a soft or hard adhesive sheet, but by means of manufacture of an adhesive sheet that has a lamination of a first adhesive layer and a second adhesive layer having different viscoelastic behaviors, and that has prescribed values of the dynamic shear storage modulus G′ in totally different temperature regions of 20° C. and 150° C.
As a means of forming such transparent adhesive sheet of the present invention, each of the first adhesive layer and the second adhesive layer is preferably formed to have a value of the dynamic shear storage modulus G′, which is measured in a 1 Hz frequency temperature dispersion, within ranges of (a) and (b) described below:
(a) G′(20° C.) of the first adhesive layer is 2×104 to 5×105 Pa, and G′(150° C.) of the first adhesive layer is 1×104 to 1×105 Pa.
(b) G′(20° C.) of the second adhesive layer is 2×105 to 5×106 Pa, and G′(150° C.) of the second adhesive layer is 5×104 to 5×105 Pa.
A transparent adhesive sheet, which has a first adhesive layer and a second adhesive layer having different viscoelastic behaviors, dominantly exhibits a viscoelastic behavior of a soft adhesive layer as a whole of the transparent adhesive sheet. That is to say, when the transparent adhesive sheet described above is measured for the dynamic viscoelasticity from the low temperature side in a shear method, the first adhesive layer having small modulus is deformed (misaligned) first, and the second adhesive layer, which is relatively hard, is hardly deformed, whereby the transparent adhesive sheet is soft overall, and the viscoelastic behavior of the first adhesive layer is exhibited dominantly. Therefore, the transparent adhesive sheet of the present invention is formed such that the first adhesive layer is softer than the second adhesive layer, and the dynamic shear storage modulus G′ of the first adhesive layer in totally different temperature regions of 20° C. and 150° C. is respectively prescribed.